The long terrh goals of this proposal are to understand the molecular mechanism by which Recql4 affects chromosome segregation during mitosis and the role of Recql4 deficiency in carcinogenesis and premature aging in mice. Aneuploidy is a hallmark of human solid tumors. While it remains uncertain whether chromosome instability plays a contributing role in tumor initiation, it is clear that it provides cancerous cells with the great adaptability to their ever changing microenvironments, including those that are brought about by therapeutic interventions. On the other hand, chromosomal instability may represent an important link between cancer and3 aging. However, the mechanisms responsible for chromosomal instability have not been fully understood. We have created a Recql4 knockout mouse model for Type II Rothmund-f homson syndrome (RTS), a cancer-prone genetic disorder caused by mutations in RECQL4. RECQL4 encodes one of the five homologues of the RecQ family of DNA helicases in humans. Recql4 knockout mice recapitulate all the major phenotypes of Type II RTS, including chromosome instability, cancer predisposition and premature aging. Our studies on this knockout mouse model have led to the discovery that premature centromere separation is the underlying cause of aneuploidy, cancer predisposition, and perhaps premature aging in these mice. Thus, these knockout mice provide a powerful tool to study a novel mechanism of chromosomal instability and a unique mechanism that links cancer and aging. Here we propose to further define the molecular mechanism by which Recql4 is involved in sister-chromatid cohesion and chromosome segregation; and the mechanism that links cancer and aging in Recql4 knockout mice. These studies will advance our understanding regarding the mechanisms that govern chromosome instability and how these mechanisms contribute to carcinogenesis and aging. Furthermore, it may also result in novel targets or strategies for treatments of cancer.